All conventional radiation detectors that are known to the inventors to be simultaneously responsive to both long wavelength infrared (LWIR) radiation and to visible radiation employ two separate detectors per detector unit (unit cell). One problem that arises from this approach is an increased fabrication complexity, detector complexity, readout complexity, interface requirement and cost. This problem is especially onerous when it is desired to provide a compact array comprised of a plurality of detector pixels or unit cells. Furthermore, it may be important that each detector unit cell or pixel be aligned or juxtaposed one with another so that there be a minimal offset between the detected IR radiation that arrives from a scene that is viewed by the array and the detected visible radiation that arrives from the same scene.
All known types of simultaneous LWIR/visible detectors that are known to the inventors do not adequately address these concerns.
Other types of dual spectral band responsive radiation detectors are known in the art, in particular detectors that are responsive to two IR radiation spectral bands.
By example, commonly assigned U.S. Pat. No. 5,113,076, "Two Terminal Multi-band Infrared Radiation Detector" to E. F. Schulte, discloses a radiation detector having two heterojunctions that function in a manner analogous to two back-to-back photodiodes. Each of the photodiodes is responsive to radiation within a different IR spectral band, such as Long Wavelength IR (LWIR) and Medium Wavelength IR (MWIR).
Also by example, commonly assigned U.S. Pat. No. 5,149,956, "Two-Color Radiation Detector Array and Methods of Fabricating Same", by P. R. Norton, teaches an array of three layer n-p-n or p-n-p photodetector structures in which a middle, electrically common layer is electrically and physically continuous across the array. The top and bottom layers are sensitive to different spectral bands, and each unit cell has two top side electrical contact structures for interfacing these layers to a readout circuit. A common contact provides a connection to the middle layer.
In general, the construction of an IR/visible detector presents a different set of problems than the construction of an IR/IR detector. This is true at least for the reason that the Group II-VI material from which these detectors are fabricated is more amenable to detecting the longer wavelength infrared radiation than the shorter wavelength visible radiation.
It is thus an object of this invention to provide a relatively simple and low cost method to fabricate an array of radiation detectors that are simultaneously responsive to both IR radiation and visible radiation.
It is another object of this invention to provide an array of radiation detectors that are simultaneously responsive to both IR radiation and visible radiation.